There’s a curious paradox at the heart of public health: When it works, nothing happens. Nobody wakes up in the morning and says, “Boy, it’s great that I don’t have cholera,” or “Isn’t it wonderful that my children aren’t crippled by polio?” Most people are only dimly aware that those diseases once decimated families, neighborhoods, and nations. In many parts of the world, they still do.

At the Johns Hopkins Bloomberg School of Public Health, we measure success by factors many people take for granted—by the absence of disease and the unremarkable good health of populations. Our ambitious mission, inherited from the School’s first dean, William Henry Welch , is captured in the deceptively simple phrase that serves as the School’s motto: Saving lives, millions at a time.

As you have seen in this special issue, we are building on an illustrious past. The School was founded by the Rockefeller Foundation in 1916 to serve as a unique model: a research institute that would accelerate the pace of scientific discovery, yoked to an educational enterprise that would train leaders from around the world to translate those discoveries into effective public health policies and programs. It has done that and more. Nearly a century later, after its first major expansion in 40 years , the Johns Hopkins Bloomberg School of Public Health leads an international network of institutions that share its mission.

Behind that mission lie the individual commitments of more than 13,000 alumni, 1,600 students from 80 countries, and 500 full-time faculty. In our own way, we’ve all experienced the epiphany of the orthopedic surgeon who left the day-to-day drama of the operating room to enroll in our Preventive Medicine Residency . “I wanted to have a greater impact,” he explained, “to prevent injuries before they happen instead of spending my time trying to put the pieces back together again.”

I understood exactly what he meant. As an ophthalmologist I knew the personal gratification of removing a blinding cataract or treating glaucoma—one patient at a time. But in Bangladesh I discovered ways to help eradicate smallpox from the face of the earth, and in Indonesia, that a 4-cent dose of vitamin A could save the sight—and lives—of over a million children every year.

Everyone at the Bloomberg School shares that breadth of purpose, whether they are developing a vaccine to combat a disease endemic in the developing world, like malaria, or discovering the cause of cervical cancer, which kills 5,000 women annually in America alone. It fuels their passion to confront the problems of malnutrition and population growth, to develop tools to predict the global spread of a new infection like SARS , or to design an effective response to an unprecedented assault, like a terrorist’s dirty bomb.

Since the attacks of 9/11, I haven’t had to explain the importance of the School’s work. But I still have to remind people of the complex, if prosaic, reality of our challenge. William Henry Welch expressed it eloquently when he said, “There are no social, no industrial, no economic problems which are not related to the problems of health.” So it remains a century later, when new infections circle the globe at the speed of jet travel, and epidemics of our own making, like diabetes, asthma, and lung cancer, claim the lives of millions.

Adapted from Dean Sommer’s foreword to Saving Lives Millions at a Time, published by the School this spring.

We are as awed by the importance of the goals that lie ahead as we are convinced of our ability to achieve them. We dream one day of ridding the world of blinding trachoma, death during childbirth, autism, and AIDS. We know that when we realize those dreams, we shall perpetuate the paradox that lies at the heart of public health: What a remarkable achievement it is when nothing happens.